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Petrogenesis of Enclaves Within the Peggy's Cove Monzogranite

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Petrogenesis of Enclaves Within the Peggy's Cove Monzogranite Imtiaz, H. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications PETROGENESIS OF ENCLAVES WITHIN THE PEGGY’S COVE MONZOGRANITE, SOUTHERN NOVA SCOTIA, CANADA HINA IMTIAZ Trinity University Research Advisor: Dr. Benjamin Surpless INTRODUCTION batholith, with the most voluminous period of intrusion taking place from 380 to 370 Ma The Canadian province of Nova Scotia is (Benn, et al., 1999). Within the batholith, the composed primarily of two tectonic terranes, Halifax pluton is a composite peraluminous the Avalonia and Meguma, juxtaposed by pluton, subdivided into three units (Fig. 1): the the Silurian – Devonian Acadian orogeny Harriet’s Field muscovite biotite monzogranite; (430–390 Ma; (Fig. 1)). Following regional he Halifax Peninsula leucomonzogranite; and metamorphism, polyphase deformation, and the Peggy’s Cove biotite monzogranite. cleavage formation throughout most of the Devonian (Keppie and Dallmeyer, 1995; Hicks The Peggy’s Cove monzogranite displays steep, et al., 1999), the thickened crust of the Meguma discordant contacts with the metasedimentary terrane experienced widespread intrusion by host rocks of the Meguma Group and is granitoids of the South Mountain Meguma NOVA SCOTIA Other terrane N AVALONIA SMB 100 km TERRANE bodies SMB Peggy’s Cove MEGUMA TERRANE Halifax Local Study Area Halifax Peninsula Harriet’s Field monzogranite Halifax Peninsula leucomonzogranite Harriet’s Field Peggy’s Cove monzogranite Halifax pluton Halifax [ Cranberry ? Head South Mountain batholith Peggy’s Cove Meguma terrane Indian Point Atlantic Ocean Local study areas Figure 1. Local study area in southern Nova Scotia. The Harriet’s Field, Halifax Peninsula, and Peggy’s Cove bodies are subunits within the larger South Mountain Batholith (SMB). All intrusive bodies have intruded the Meguma terrane. Cran- berry Head, Peggy’s Cove, and Indian Point localities are within the Peggy’s Cove monzogranite, adjacent to the intrusive contact with the Meguma terrane. (modified from Keppie, 2000) 255 Imtiaz, H. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications thypothesized to be the earliest crystallizing higher mafic content relative to the host rock unit of the Halifax pluton (e.g., MacDonald and with both darker (slightly more mafic) enclaves Horne, 1988). The hypothesized petrologic (4-8 cm in diameter) and megacrystic feldspars origin of the monzogranite has been determined concentrated along the boundaries of the larger (e.g., MacDonald and Horne, 1988), but little bodies and in the centers of the smaller bodies. attention has been paid to enclaves within the unit. The purpose of this study is to determine ENCLAVE PETROGRAPHY the petrogenetic origin of enclaves within the Peggy’s Cove monzogranite using field Initially, bodies of mafic appearance within relationships, thin section petrography, whole- the Peggy’s Cove monzogranite were called rock geochemistry, and microprobe analyses. enclaves to avoid genetic interpretation prior to petrographic, geochemical, and microprobe FIELD RELATIONSHIPS analyses. The enclaves have been divided into five types based on how distinct the The Peggy’s Cove monzogranite crops out petrographic textural features of each enclave along the Atlantic Ocean near St. Margaret’s were relative to the host rock as well as relative Bay, just south of Halifax (Fig. 1). Based on to other enclaves (Table 1): exposed field relationships, it is likely that the intrusive contact between the monzogranite Type 1: Fresh to moderately-assimilated and the Meguma is just offshore in the Atlantic metasedimentary xenoliths. These enclaves Ocean. While the most abundant enclave types tend to have angular to sub-angular, discrete were sampled for this study, some enclave boundaries, suggesting brittle deformation. types observed in the field were not sampled The fresh metasedimentary bodies commonly but provide important information for models exhibit features found in Meguma Group rocks, of enclave petrogenesis. These include: (1) including metamorphic textures and mineralogy enclaves composed of spherical clumps of (e.g., andalusite and sillimanite). The garnet- nearly pure biotite (~1 cm in diameter) with ~ bearing biotite clumps described above also fall 0.5 cm garnet porphyroblasts, usually centered into a Type 1 category. The more assimilated in the enclave or clotted together on the enclave bodies consist of 40-50% subhedral biotite, margin; (2) large bodies (from ~40-50 cm to ~2- 40-50% subhedral muscovite, 3-5% anhedral 3 m in diameter) of finer grain size and slightly plagioclase, 3-5% anhedral potassium feldspar, 3-5% anhedral quartz, and trace amounts of Table 1. Enclave types. secondary minerals (e.g., chlorite). Type Distinguishing Characteristics Possible Origin primary sedimentary features; angular to sub-angular boundaries; and 1 xenolith metamorphic textures and mineralogy mineralogy dominated by micas; strongly folded foliation; acicular apatite; 2 mixed/mingled magma and chilled margins fine-to medium-grained; porphyritic (abundant medium-to coarse-grained 3 megacrysts of quartz and feldspars); acicular apatite; and gradational mixed/mingled magma boundaries strong compositional banding (defined by mica-rich and quartz/feldspar 4 autolith zones); and chilled margins fine-to medium-grained; lower mafic content than host rock; generally equal 5 autolith dimensions; and gradational boundaries 256 Imtiaz, H. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications Type 2: Fine-to medium-grained enclaves with a mineralogy dominated by micas, exhibiting strong folded foliation (Fig. 2c) and higher Plag xenocryst mafic content than the host rock. This enclave ranges from 4-5 cm in diameter and display 0.5 mm chilled margins. The biotite and muscovite define the foliation with 70-75% subhedral biotite, 15% subhedral muscovite, 4% 1 cm anhedral quartz, 3% subhedral plagioclase, 2% subhedral potassium feldspar, and trace amounts b of secondary minerals. Quartz and feldspar crystals contain acicular apatite crystals with length-to-width ratios up to 60:1 (Fig. 2d). Plag Type 3: Fine-to medium-grained porphyritic enclaves with medium-to coarse-grained megacrysts of quartz and feldspars and bulk 0.5 mm composition (Fig. 2a) similar to the host rock. Enclaves range from 4-9 cm in diameter, c are more equant than other enclave types, and display gradational boundaries, with monotonous changes in both mineralogy and grain size across a 1-4 mm boundary zone. They consist of 30-48% subhedral plagioclase with disequilibrium textures (Fig. 2b), 5-45% anhedral quartz, 2-30% subhedral biotite, 1-35% 1 cm subhedral potassium feldspar, 1-2% subhedral muscovite, and trace amounts of secondary minerals (e.g., chlorite). Acicular apatite d crystals are present as inclusions in quartz and feldspar and exhibit length-to-width ratios up to 60:1. Type 4: Fine-to medium-grained enclaves with strong compositional banding. These enclaves 0.4 mm range from 10–20 cm in length, with a common Figure 2. Photomicrographs of important length-to-width ratio of 4:1 or 5:1. These features within enclaves. (a) Enclave Type 3 enclaves have chilled margins, with monotonous –relatively sharp enclave–host rock boundary changes in both mineralogy and grain size with a highly altered plagioclase xenocryst inclusion within the finer-grained enclave. across a 0.5-1.5 mm boundary zone. The [Crossed polars.] (b) Enclave Type 3 – plagio- compositional banding is defined by mica-rich clase crystals with cores that exhibit disequilib- rium textures. [Backscattered image from UT (biotite >> muscovite) and quartz-feldspar- rich Austin electron microprobe facility.] (c) zones. Within each compositional band, there Enclave Type 2 – strong folded foliation, defined primarily by biotite and muscovite. is no preferred alignment of micas or feldspars [Crossed polars.] (d) Enclave Type 2 – acicular relative to the orientation of the banding, which apatite present as inclusions within quartz and is in strong contrast to Type 2 foliations. These feldspar. [Uncrossed polars.] 257 Imtiaz, H. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications enclaves consist of 40-45% subhedral biotite, ENCLAVE GEOCHEMISTRY 25-40% subhedral plagioclase, 0-30% subhedral potassium feldspar, 1-11% anhedral quartz, 1- Major, minor, and trace element geochemical 2% subhedral muscovite, and 2-3% subhedral data from host rock and enclave samples are chlorite. displayed in Figure 3. These data consist of nineteen analyses of the South Mountain Type 5: Fine-to medium-grained equigranular batholith and five analyses of enclaves within granitoid enclaves with granitic texture and in the Peggy’s Cove monzogranite (two Type 3 lower mafic mineral content than the host rock. samples and three samples from other enclaves These enclaves range from 4-6 cm in diameter, within the monzogranite). Major and minor are approximately equant in dimension, element data suggest a liquid line of descent, but and display gradational boundaries with the not all enclave samples are related to the South host rock, with monotonous changes in both 7 mineralogy and grain size across a 1.5-5.0 mm 6 boundary zone. These bodies consist of 30- MgO (%) 5 45% sub-to anhedral quartz, 20-30% subhedral 4 Enclaves biotite, 15-25% subhedral plagioclase, 4-25% SMB subhedral potassium feldspar, <1-15% subhedral 3 chlorite, and <1% subhedral muscovite. 2 The significance of acicular apatite 1 Wyllie and others (1962) experimentally 0 determined that the habit of apatite crystals
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